WASHINGTON — President Barack Obama warned Friday that it's "way too early to be optimistic," and a top BP executive also cautioned against over-reacting to BP's latest effort to contain the oil that's been spewing from a broken well a mile under the Gulf of Mexico.

The company will know for certain in coming days whether it's fully contained the runaway well and how much oil it's able to capture, BP Senior Vice President Kent Wells said.

Although oil is still flowing from the containment device the company put in place Thursday evening, the flow of oil that's visible via underwater cameras is expected to disappear as crews close the cap's vents and begin sending more of the oil to a ship on the surface. The process is slow because engineers are concerned about crystals known as hydrates forming in the cap, which doomed one of the early efforts at containing the well.

Wells said that while he was "quite encouraged" by the progress, he "wouldn't want to oversell or undersell what we might be able to do.

"I do believe it will take us a few days to get up to peak efficiency. And I think we'll know more over the next couple of days of what we could ultimately get to."

It was rare good news 45 days after an explosion killed 11 people and sent as much as 25,000 barrels of oil a day into the fragile Gulf ecosystem. Even so, like the president and BP officials, the national incident commander for the oil spill cautioned against too much optimism.

"Generally, progress is being made," retired Coast Guard Adm. Thad Allen said, adding, "I think we need to caution against over-optimism here. There's always been adjustments that have been made in the process as we move forward, but in general, progress is being made."

News of the containment came as tar balls began to wash up on the beaches of Pensacola, Fla., and people along the coasts of Alabama, Mississippi and the Florida Panhandle braced for an oily invasion and struggled to share the resources needed to fend it off.

"We now have a battle line, if you will, that stretches from Terrebonne Parish in Louisiana over to around the beaches by Pensacola, Florida," Allen said.

Obama, in his third visit to the region, slammed BP on Friday for spending a reported $50 million on TV advertising to improve its image, and he criticized its plan to pay out $10.5 billion in dividends this quarter.

While the president said he didn't fault BP for making good on obligations to shareholders, he added that the company has "moral and legal obligations here in the Gulf for the damage that has been done. And what I don't want to hear is that when they're spending that kind of money on their shareholders and spending that kind of money on TV advertising, that they're nickel-and-diming fishermen or small businesses here in the Gulf who are having a hard time."

Less than a mile from where Obama met with residents in Grand Isle, La., pools of thick brown oil that resembled chocolate pudding covered at least one beach. Offshore, on Queen Bess Island, a nature refuge, pelicans appeared unable to fly because their wings were matted with oil.

The president will greet family members of those killed on the Deepwater Horizon next Thursday at the White House to offer his condolences in person, Press Secretary Robert Gibbs said Friday. Obama sent letters to each family with a White House invitation.

BP officials said Friday that they'd wait until they knew more about the flow rate before disclosing how much oil their containment device was collecting. The company has shied away from government estimates that at least 12,000 to 25,000 barrels of oil have been escaping each day from the mangled well at the bottom of the ocean.

BP's Wells did say that the company's drill ship on the Gulf's surface, the Discoverer Enterprise, is capable of processing 15,000 barrels of oil a day. BP is working on diverting some of the flow to another vessel, too.

"Everybody wants us to collect every barrel we can. Clearly, that is our objective," Wells said.

The oil will be sent by tanker to refineries and sold. The value of about one in five barrels the company produces is owed to the U.S. Treasury in the form of royalties.

The government-appointed Flow Rate Technical Group will meet Monday to refine its estimates. So far, it hasn't been able to give a top-end estimate, but at least one group member said it was probably much larger than the government's initial estimate.

The group thought it was "irresponsible and unscientific" to suggest an upper limit because it's working with limited data provided selectively by BP, said Ira Leifer, an associate researcher at the Marine Science Institute of the University of California, Santa Barbara, who's a member of the group.

The initial videos they were working from were "YouTube quality," said James Riley, a professor of mechanical engineering at the University of Washington who's also a group member. Later tapes were of better quality, he said.

The researchers said it was difficult to determine the ratio of oil to gas in the flow on the basis of the videos.

"If we can develop a good technique to analyze this, someone will probably eventually look at all the tapes," he said. "We can't look at a month of data. We're trying to determine an average daily amount based on limited records."

Scientists would like to see data that were gathered independently, Leifer said. He also said it would be possible for researchers to capture samples from the flow using remote operating vehicles. That would help them determine the oil-to-gas mix and come up with a number that isn't reliant on BP's video feeds.

"There's frustration about the absence of independent scientific data not filtered through BP in some manner," Leifer said.

An accurate spill measurement is important because it would have a number of financial consequences for BP. The amount spilled is used as evidence when federal investigators consider fines, and if juries impose damages on BP for the spill. BP also is responsible for paying royalties for each barrel of oil lost.

Knowing how much oil is emerging from the well is important for engineering reasons, too, and because the company also is expected to pay royalties on the oil it's collecting.

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